According to the World Health Organisation (WHO) the current guideline was set to protect the public from the health effects of gaseous pollution.

“At short-term concentrations exceeding 200 μg/m3, it is a toxic gas which causes significant inflammation of the airways,” it said.

“NO2 is the main source of nitrate aerosols, which form an important fraction of PM2.5 and, in the presence of ultraviolet light, of ozone.

“The major sources of anthropogenic emissions of NO2 are combustion processes (heating, power generation, and engines in vehicles and ships).

“Epidemiological studies have shown that symptoms of bronchitis in asthmatic children increase in association with long-term exposure to NO2. Reduced lung function growth is also linked to NO2 at concentrations currently measured (or observed) in cities of Europe and North America,” it said.